ELECTRICITY. 79 



will present a different appearance from that which is beyond 

 its reach, and a whitish film, somewhat similar to that seen on 

 the mercurialised portions of a daguerreotype, will gradually 

 appear on the portion of the plate affected by the discharge. 

 If the gas be a compound, as carbonic oxide, or a mixture, 

 as oxygen and hydrogen, and consequently contain elements 

 capable of producing oxidation and reduction, then the effect 

 upon the plate will depend upon whether it be positive or 

 negative ; in the former case it will be oxidated, in the latter 

 the oxide, if existing, will be reduced. This effect will also 

 take place in atmospheric air, if it be highly rarefied, and 

 can hardly be explained otherwise than by a molecular polar- 

 isation of the compound gas, If, again, the metal be reduced 

 to a small point, and be of such material that the gas cannot 

 act chemically upon it, it can yet be shown to be disintegrated 

 by the electric spark. Thus, let a fine platinum wire be her- 

 metically sealed in a glass tube, and the extremity of the tube 

 and the wire ground to a flat surface, so as to expose a section 

 only of the wire ; after taking the discharge from this for some 

 time, it will be found that the platinum wire is worn away, 

 and that its termination is sensibly below the level of the glass. 

 If the discharges from such a platinum wire be taken in gas 

 contained in a narrow tube, a cloud or film consisting of a 

 deposit of platinum will be seen on the part of the tube sur- 

 rounding the point. 



Another curious effect which, in addition to the above, I 

 have detected in the electrical discharge in attenuated media, 

 is that when passing between terminals of a certain form, as 

 from a wire placed at right angles to a polished plate, the dis- 

 charge possesses certain phases or fits of an alternate character, 

 so that, instead of impressing an uniform mark on a polished 

 plate, a series of concentric rings is formed. 



Priestley observed that, after the discharge of a Leyden 

 battery, rings consisting of fused globules of metal were formed 

 on the terminal plates ; in my experiments made in attenu- 

 ated media, alternate rings of oxidation and deoxidation are 

 formed. Thus, if the plate be polished, coloured rings of 

 oxide will alternate with rings of polished or unoxidated sur- 

 face ; and if the plate be previously coated with an uniform 



